Currently there is a need for precise attachments of miniature light-weight plastic pieces to each other. For example, in the emerging field of microfluidics, particularly in labware, connecting channels and reservoirs are used for effecting chemical reactions and assays on a scale involving microliters of fluids. Microfluidic devices allow for self-contained reactions within a single, light-weight reaction container. Advantages of this system include an ability to perform microscale assays and reactions, saving material costs, and the development of self-contained systems that minimize exposure to potentially hazardous materials. To effect this technology requires precise and reliable attachment of plastic parts. Manufacturing such parts necessitates precise attachment of small plastic pieces.
One method used to attach together plastic pieces is ultrasonic welding. As with adhesives, ultrasonic welding enables formation of a strong, hermetic seal along the length of a weld seam. However with ultrasonic welding, unlike with adhesives, plastic pieces can be attached together without the problems stemming from drying adhesives, such as outgassing or bleeding. In ultrasonic welding an "energy director" on a plastic piece is used to focus the weld energy. The energy director is a protrusion or bead which runs along the length of a seam to be ultrasonically welded, causing localized melting of the energy director. Since the energy director is usually a solid, ultrasonically conductive polymer, typically a thermoplastic material, such as acrylic, it melts when ultrasonic energy is applied and the melted plastic protrusion bonds to an abutting second plastic piece which partially melts. U.S. Pat. Nos. 5,401,342 and 5,540,808 to Vincent et al. teach the use of two angled ridges as energy directors. The shape of the energy director promotes thermoplastic flow as two plastic pieces are attached together.
U.S. Pat. No. 5,782,575 to Vincent et al. teaches that an energy director may have first and second parallel ridges, with a groove therebetween. A ledge adjacent one of the ridges is provided. The ledge is included, despite its tendency to cause undesirable flashing, to help stop the welding before the plastic parts melt through.
Currently by using a standard angled energy director, the energy from an ultrasonic welding horn focuses energy on a small area causing heating and melting of thermoplastic ridges, beads or protrusions. It is difficult to stop this process at a precise depth. This precludes the ability to use ultrasonic welding for attachment of plastic pieces at an exact height, as would be needed to attach two plastic pieces together to form an interior cavity of precise dimensions. Using standard technology limitation of weld depth can be effected by limiting the amount of welding energy or mechanical limitations. This results in weld depth precision of plus or minus 10 microns. This tolerance is too inaccurate for the formation of interior cavities with precise dimensions.
The object of this invention is to provide a simple method to more accurately control ultrasonic weld depth.